Work - energy theorem and conservation of momentum question

There is a hemispherical track of radius R = 4m. A block B is lying at rest at the bottom of the track. A block A is pushed along the horizontal portion of the track with speed u. Collision between A and B is perfectly elastic. Find u such that B reaches P (the end of hemispherical portion of track). (See figure for clarity)

Staff: Mentor

OK, I see what you did. Makes sense.

The problem is that you found the pre-collision speed of A at the bottom of the track, but they want the speed while it's still on the horizontal portion. Redo it with that in mind and you'll get the right answer.